1. Field of the Invention
The present invention relates to a defect detecting device for detection of any defective pixel (spot) or photo sensing element in a solid-state image sensor, and more particularly to a device for detecting defects in a solid-state image sensor having an image pickup area larger than an effective image pickup area.
2. Description of the Related Art
In a solid-state image sensor composed of a semiconductor such as a CCD (Charge Coupled Device), there may be induced some defective pixels (photo sensing elements) where the sensitivity is lowered due to local crystal defects or the like of the semiconductor, or some other defective pixels derived from flaws and so forth. It is known that, if any of such pixel defects is existent, the picture quality is deteriorated by the image pickup output of the relevant defective pixel. Particularly in a CCD solid-state image sensor, white spot defects and the like are caused as pixel defects. And even in a dark condition where none of light is incident on a CCD solid-state image sensor, each white spot defect is observed in dimensional unit of a pixel on a screen.
A white spot defect, which is a subject to be detected and corrected in a CCD solid-state image sensor, is extremely low in level, and even a defect level of several mV or so at normal temperature raises a problem in ordinary reading. Since the level of a pixel defect that usually causes a problem is so extremely low, it is impossible to achieve simple detection of a defective pixel. However, if the signal is amplified to raise the detection sensitivity, the noise is also amplified together to consequently deteriorate the S/N. One of the methods proposed heretofore to enhance the detection sensitivity while retaining the S/N sufficiently high is carried out by forcibly increasing the time, which is predetermined for storage of signal in each photo sensing element (pixel), to a length far greater than a normal one. More concretely, at detection of a defect, the storage time is prolonged to ten and several fields in a frame read mode.
Meanwhile, for correcting a wobbling fault of a video camera derived from its unsteady manual hold, there is known a method of employing a solid-state image sensor which has an image pickup area larger than an effective image pickup area required in a normal image sensing mode, and using a portion of such image pickup area for correction of the wobbling fault derived from unsteady manual hold of the video camera. When detection of defective pixels is performed in a solid-state image sensor equipped with such a wobbling-fault correcting function, a time corresponding to two fields is allocated to the detection, since a time corresponding to more than one field is required for detecting defective pixels in one field due to the fact that the number of vertical lines is greater than that in the ordinary TV system.
FIG. 5 shows the timing to drive an exemplary CCD solid-state image sensor in the related art and the waveform of a CCD output signal. In this example, a storage time is set to a period corresponding to six fields. In FIG. 5, FLD stands for an ODD/EVEN field discrimination signal, VDI for a vertical sync pulse, VDO for a read timing pulse synchronized with the vertical sync pulse, XSG1 for a read pulse superimposed on a first-phase vertical transfer pulse V1 of four-phase vertical transfer pulses V1-V4 to read out the signal charge of one field, XSG2 for a read pulse superimposed on a third-phase vertical transfer pulse V3 to read out the signal charge of the other field, and CCDout for a CCD output signal, respectively.
In FIG. 5, a read pulse XSG2 is generated at time point t1 to read out an even field, and then storage is performed with regard to this EVEN field for a period corresponding to six fields. Meanwhile with regard to an ODD field, a read pulse XSG1 is generated at time point t2 to read out the odd field once, and then storage is performed for a period corresponding to six fields from time point t3 after a lapse of a period corresponding to two fields, since a time of more than one field is required, as mentioned, for detection of defective pixels in one field, and further the pixel reading order needs to be rendered identical.
Subsequently a read pulse XSG2 is generated at time point t4 when the storage period corresponding to six fields is terminated with regard to the EVEN field, whereby the EVEN field is read out. And detection of defective pixels is executed for a period of more than one field with regard to the even field on the basis of the pixel information thus read out. Meanwhile with regard to the ODD field, the storage period corresponding to six fields is terminated at time point t5 after a lapse of a period corresponding to three fields from time point t4 when the even field is read out, so that the ODD field is read out at this time point t5, and detection of defective pixels with regard to the ODD field is executed on the basis of the pixel information thus read out.
However, in the CCD solid-state image sensor equipped with the wobbling-fault correcting function mentioned above, a long time of more than one field is required for detection of defective pixels in one field due to the fact that the number of vertical lines is larger than that in the ordinary TV system, whereby a time period corresponding to two fields is allocated to such detection. Furthermore, since it is necessary to read out EVEN and ODD fields alternately, a time period corresponding to three fields is required for reading out the next field, hence necessitating a period of five fields in total for the detection.
Normally the process of detecting and correcting defective pixels is executed, in most cases, at the time of initial operation when the power supply for the camera is switched on, so that such process restricts the time until appearance of a picture on the camera after switching on the power supply. Since it is usually necessary to display a picture immediately after the power supply is switched on, the time required for executing the initial operation needs to be minimized, and the initial operation should be performed promptly. Therefore the period required for detection of defective pixels also needs to be minimized, but the detection sensitivity is lowered if the storage time is shortened for reducing the detection period.